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What differentiates the Transmission
Control Protocol from simpler transport protocols like UDP is the quality
of the manner in which it sends data between devices. Rather than just
sticking data in a message and saying off you go, TCP carefully
keeps track of the data it sends and what happens to it. This management
of data is required to facilitate two key requirements of the protocol:

Reliability: Ensuring that data that is
sent actually arrives at its destination, and if not, detecting this
and re-sending the data.

Data Flow Control: Managing the rate at
which data is sent so that it does not overwhelm the device that is
receiving it.

To accomplish these tasks, the entire
operation of the protocol is oriented around something called the sliding
window acknowledgment system. It is no exaggeration to say that
comprehending how sliding windows works is critical to understanding
just about everything else in TCP. It is also, unfortunately, a bit
hard to follow if you try to grasp it all at once, which means many
people's eyes glaze over trying to make sense of it.

Since you cant really get
TCP without understanding sliding windows, I wanted to make sure that
I explained the mechanism thoroughlyand without assuming you already
understand a great deal, as most references do. For this reason I am
going to start with the concepts behind sliding windows and eventually
explain how the technique works in general terms and why it is so powerful.
Doing this properly required a considerable amount of explanation (which
took a long time to get right, I might add!) so buckle your seat belt.
J

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